The flue gas of furnaces and boilers, such as those used in power generation plants, carries matter including ash and other particulates which pollute the atmosphere. Electrostatic precipitators are used to remove ash and other particulates carried in the flue gas. Electrostatic precipitators operate by causing the individual particles in the flue gas to accept an electrical charge and by attracting the charged particles to collector plates for disposal.
Electrostatic precipitation has been used primarily in connection with the burning of coal. As coal burns, it produces H.sub.2 O, CO.sub.2, CO, SO.sub.2, SO.sub.3, ash and other particulate matter and products of combustion. The H.sub.2 O and SO.sub.3 combine to form H.sub.2 SO.sub.4 (sulfuric acid) which coats the particulate matter. The coating of H.sub.2 SO.sub.4 reduces the resistance of the ash and other particulate matter and thereby facilitates the electrical charging of this particulate matter so that the charged particulate matter can be more easily attracted to the collector plates of the electrostatic precipitator. If combustion produces insufficient H.sub.2 SO.sub.4, however, the resistance of the particulate matter is high which reduces the efficiency of the electrostatic precipitator in charging the particulate matter suspended in the flue gas and, as a result, in collecting particulate matter from the flue gas.
When coal having a relatively high sulfur content is burned, sufficient SO.sub.3 is produced to form the proper amount of H.sub.2 SO.sub.4. However, high sulfur coal also produces excess SO.sub.2 which, if exhausted to the atmosphere, is a pollutant that has been linked to acid rain.
In order to reduce SO.sub.2 emissions, the operators of coal fired boilers and furnaces have burned coal having a low sulfur content. However, low sulfur coal results in the production of less SO.sub.3 than that required to efficiently operate the electrostatic precipitators. Accordingly, one must balance the need for lower SO.sub.2 emissions and the need for an adequate supply of SO.sub.3 to maintain the efficiency of electrostatic precipitators at a relatively high level. To provide this balance, the operators using lower sulfur content coal have injected a controlled amount of SO.sub.3 into the flue gas to compensate for the inadequate amount of SO.sub.3 produced by combustion of the low sulfur coal. Thus, SO.sub.2 emissions are held relatively low while electrostatic precipitator efficiency is increased.
As is apparent, the efficiency of some electrostatic precipitators is dependent upon the concentration of SO.sub.3 in the flue gas. That is, if the SO.sub.3 concentration in the flue gas is too low, an electrostatic precipitator may operate at less than optimal efficiency and an unacceptable plume of particulates may result. Flue gas that has less than optimal SO.sub.3 concentrations as the flue gas enters the electrostatic precipitator constitutes an emissions problem. Coal fired power generation plants that are operating out of compliance with emission regulations can be forced to reduce their power output until the emissions are brought back into compliance. Accordingly, it is important to keep the emissions concentrations within an acceptable range while minimizing the power consumption of the electrostatic precipitator.
One prior art method of decreasing the power consumption of an electrostatic precipitator is to measure the opacity of the flue gas as it exits from a stack of the flue gas conditioning system and to control the amount of power supplied to the electrostatic precipitator accordingly. For example, Reese, et al., U.S. Pat. No. 4,284,417 discloses a system for controlling the electric power supplied to an electrostatic precipitator having an opacity-sensitive transducer which produces an output signal proportional to the opacity of the flue gas exiting from the precipitator. The system also includes a comparator which compares the output signal with preset upper and lower limits and a controller which controls the power supplied to the precipitator in order to restore the flue gas opacity to a permissible range when the output signal falls outside of the preset upper and lower limits. Krigmont, et al., U.S. Pat. No. 4,987,839 discloses a system including a source of SO.sub.3 which adds SO.sub.3 to flue gas before it enters an electrostatic precipitator and a controller which controls the rate at which the SO.sub.3 is added to the flue gas. The controller is responsive to the opacity of the flue gas exiting the electrostatic precipitator and to the power supplied to the electrostatic precipitator.
Measuring the opacity of the flue gas as it leaves the flue gas conditioning system, however, is not necessarily the best method of controlling the level of ash and other particulates in flues because the opacity of the flue gas is not a good indicator of the need for the addition of a particular additive, such as SO.sub.3.
Another method of increasing the efficiency of an electrostatic precipitator is to employ, as a control for the amount of SO.sub.3 delivered to the flue, the power delivered to the electrostatic precipitator. A system employing this method is disclosed in Woracek, et al., U.S. Pat. No. 4,779,207, wherein a flue gas conditioning system includes automatic voltage controllers (AVCs) which supply power to transformer/rectifier sets which, in turn, provide a stepped-up and rectified voltage to elements or plates of an electrostatic precipitator. Power measuring elements produce signals indicative of the power delivered by each of the AVCs to each of the transformer/rectifier sets, and these signals are combined to produce an indication of the average power delivered to the electrostatic precipitator. The average power indication is used to control the amount of SO.sub.3 delivered to the flue so as to keep the average power delivered to the electrostatic precipitator within a predetermined range.
It is also known in the prior art to intermittently energize the elements of an electrostatic precipitator, which may include plates, electrodes and the like, in order to increase the electrostatic precipitator efficiency. A flue gas conditioning system which is utilized for an electrostatic precipitator system with intermittent energization typically has an AVC which supplies an intermittent voltage, having a predetermined duty cycle, to a transformer/rectifier circuit which, in turn, provides a stepped-up, rectified, intermittent voltage to the electrostatic elements. Krigmont, et al., U.S. Pat. No. 4,987,839 discloses an intermittently energized system having a control which is responsive to the duty cycle of the power delivered to an electrostatic precipitator and which uses this duty cycle to estimate the power delivered to the flue gas by the electrostatic elements.
Intermittent energization, however, creates a control problem in flue gas conditioning systems like that disclosed in the Krigmont, et al. patent and those which control the amount of SO.sub.3 delivered to the flue in accordance with a power signal developed from the duty cycle of the intermittent power supplied to the electrostatic precipitator. This problem occurs because neither (a) the duty cycle of the intermittent power nor (b) the actual power developed by the precipitator power source with which the flue gas conditioning system is used, are reliable indications of the power actually delivered to the flue gas by the electrostatic elements. As such, an average power signal developed from the duty cycle of the intermittent power is an inaccurate determination of the amount of SO.sub.3 which needs to be provided to the flue gas entering the electrostatic precipitator. This problem becomes particularly acute when the duty cycle of the power source is periodically changed during operation of the flue gas conditioning system. In such a system, therefore, other means must be provided for measuring the power absorbed by the electrostatic elements of the electrostatic precipitator.